1. Field of the Invention
The present invention relates to an electro-optic sampling oscilloscope used in various signal measurements.
This application is based on patent appellation No. Hei 10-122514 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In recent times, electro-optic sampling oscilloscopes are favored that can measure ultra-fast bit-rate signals of the order of 2.4 Gbps on a target circuit, without disturbing the operation of the target circuit.
In such an electro-optic sampling oscilloscope, an electro-optic probe based on electro-optic effects is used to detect signals in the target circuit.
Such electro-optic sampling oscilloscopes are favored for measurements in communication technologies that are constantly evolving into ultra-fast systems, because of the following features of the device.
(1) Measurement process is facilitated because the technique does not required a ground line.
(2) Metal pin placed at the end of an electro-optic probe is insulated from the test circuit resulting an extremely high input impedance, so that measurement process hardly affects the performance of the target circuit.
(3) Because optical pulses are used, measurements are possible over a wide bandwidth of the order of GHz.
FIG. 5 is a schematic side view showing the components of an electro-optic probe used in a conventional electro-optic sampling oscilloscope. An electro-optic probe 4 is based on a principle that when an electro-optic crystal, that is being subjected to an electrical field generated by the target signal, is irradiated with a laser beam, the polarization state of the laser beam is altered.
In the electro-optic probe 4, a metal probe 5 with a tapered tip touches a signal line in a circuit. The metal pin 5 serves the purpose of facilitating the electrical field of the target signal to affect the condition of the electro-optic (e-o) crystal 7. A circular shaped insulator 6 is provided to contact the end of the metal pin 5 in the center of the rear surface of the insulator. In other words, the metal pin 5 is surrounded by the insulator 6. The e-o crystal 7 is a cylindrical crystal of BSO (B.sub.12 SiO.sub.20), and has a property, known as the Pockels' effect, that the primary opto-electric effect, which is its refraction index, is altered in response to an electrical field coupled through the metal pin 5.
A reflection mirror 8 is a dielectric film laminated mirror and is made by vapor deposition of a reflecting substance on the rear surface of the e-o crystal 7. A reference laser beam La0 transmitted through the e-o crystal 7 is reflected by the reflection mirror 8, which is bonded to the front surface of the insulator 6.
A cylindrical casing 9 is comprised by a tube section 9b and an end piece 9a of a tapered-shape integrally formed at one end of the tube section 9b having a hole through the axial center. The end piece 9a houses the metal pin 5, insulator 6, e-o crystal 7 and the reflection mirror 8.
An optical fiber 10 is a polarization-maintaining optical fiber, and connects a connector 11 and a laser generator (not shown). The laser generator generates a linearly polarized reference laser beam La0. The reference laser beam La0 is comprised by base band component signal that does not contain signal components in the measurement band. The connector 11 is disposed so that the reference laser beam La0 output from the ejection end 11a will be injected at right angles to the e-o crystal 7 and the reflection mirror 8. A collimator lens 12 is disposed on the left of the connector 11, and converts the reference laser beam La0 to a parallel beam of light.
A polarized beam splitter 13 is disposed on the left of the collimator lens 12, and transmits a polarized component of the reference laser beam La0 parallel to the plane of the paper in a straight line, while the polarized component of the reference laser beam La0 is bent at 90 degrees to the plane of the paper, and the bent beam is transmitted as the second signal light La2 in a straight line. A Faraday element 14 is disposed on the left of the polarized beam splitter 13, and rotates the polarized component of the reference beam La0, transmitted through the polarized beam splitter 13, at 45 degrees to the plane of the paper.
A half-wave plate 15 is disposed on the left of the Faraday element 14 in such a way that the orientation of its crystal axis in inclined at 22.5 degrees, and re-directs the polarized beam rotated by the Faraday element 14 in a direction parallel to the plane of the paper. A polarized beam splitter 16 is disposed on the left of the half-wave plate 15, and has the same structure as the polarized beam splitter 13, and splits a portion of the reference laser beam La0 reflected from the reflection mirror 8 as the first signal light La1. A full-wave plate 17 is disposed on the left of the polarized beam splitter 16, and adjusts the S/N (signal to noise) ratio of the output signals ultimately obtained from the e-o probe 4, by adjusting the intensity balance of the reference laser beam La0 transmitted through the polarized beam splitter 16. Adjustment of S/N ratio is performed by varying the angle between the reference laser beam La0 and the wave plate 17 by rotating the wave plate 17.
A first photo-diode 18 is disposed above the polarized beam splitter 16, and converts the first signal light La1 (a portion of the reference laser beam La0 split by the polarized beam splitter 16) into first electrical signals and outputs the electrical signals to a positive (+) terminal of a differential amplifier 30. A second photo-diode 19 is disposed above the polarized beam splitter 13, and converts the second signal light La2 (a portion of the reference laser beam La0 split by the polarized beam splitter 13) into second electrical signals and outputs the electrical signals to a negative (-) terminal of the differential amplifier 30.
In such an apparatus, when the metal pin 5 shown in FIG. 5 is made to contact a signal line (not shown), an electrical field of a magnitude, corresponding to the level of the signal in the target circuit, propagating in the signal line, and couples with the e-o crystal 7. Accordingly, refraction index of the e-o crystal 7 changes with the strength of the electrical field. In this condition, a reference laser beam La0 is injected into the front surface of the e-o crystal 7, through the output end 11a of the connector 11, collimator lens 12, polarized beam splitter 13, Faraday element 14, 1/2 wave plate 15, polarized beam splitter 16 and the wave plate 17.
Under this condition, the polarization state of the reference laser beam La0 propagated through the e-o crystal 7 is changed. Polarization-affected reference laser beam La0 is reflected from the reflection mirror 8, and is output from the front surface of the e-o crystal 7, and is separated in the polarized beam splitter 16. The first signal light La1 produced by this splitting process is converted into first electrical signals in the first photo-diode 18, and the first electrical signal S1 are input in the (+) terminal of the differential amplifier 30.
In the meantime, the second signal light La2 produced by the polarized beam splitter 16 is diverted by the polarized beam splitter 13 to the second photo-diode 19, and is converted into second electrical signal S2 in the second photo-diode 19, and the second electrical signal S2 are input in the (-) terminal of the differential amplifier 30.
Accordingly, the first and second electrical signals S1, S2 are amplified in the differential amplifier 30, in such a way that the in-phase noise components contained in the reference laser beam La0 generated by fluctuation and other factors are canceled.
Differentially amplified signals are input as detection signal SO of the e-o probe 4 into the input terminal of the sampling oscilloscope.
The result is a display of the waveform of the signals transmitting in the signal line on the display section of the sampling oscilloscope.
It was mentioned above that in the conventional e-o sampling oscilloscope, noise components are canceled in the differential amplifier 30. However, it presupposes that the noises in the first electrical signal S1 and that in the second electrical signal S2 are at the same level.
In practice, however, the noise level contained in the electrical signal S1 (first signal light La1), and the noise level contained in the second electrical signal S2 (second signal light La2) are different because of the following reasons, so that the S/N ratio in the detection signal S0 can be rather high. Some of the reasons are:
(1) Optical properties of the optical components and electrical properties of electrical components are altered due to external factors such as temperature variations,
(2) Optical paths for propagation of the first signal light La1 and the second signal light La2 are affected by vibration and pressure and the like, and
(3) Electrical properties of the first photo-diode 18 and second photo-diode 19 are not uniform.
To counter such problems, conventional e-o sampling oscilloscopes provide a manual adjustment device to adjust the orientation of the wave plate 17 to improve the S/N ratio in the detection signal S0. However, this process is delicate and requires considerable experience for proper adjustment, and furthermore, a special adjustment device is required for the wave plate 17.
Additionally, in the conventional e-o sampling oscilloscopes, even if the S/N ratio of the detection signal S0 is improved, it still leaves the problem of degrading S/N ratio caused by external factors.
Therefore, the conventional e-o sampling oscilloscope present a cumbersome problem that whenever S/N ratio is degraded, it is necessary to manually adjust the wave plate 17.